Physical Properties of Graphite

Graphite is grey and lead-like in appearance; it possesses a metallic lustre, and is opaque even in the thinnest flakes.

It crystallises naturally and from molten iron, in hexagonal plates belonging to the monoclinic system. It is greasy to the touch, and its hardness on Moh's scale is less than 1, that of diamond being 10. It is on account of its softness, and because it rubs off in little scales, that it leaves a streak on paper; and it is the same property that makes it useful as a lubricant. Indeed, according to Threlfall, it behaves under compression like a liquid. It is porous to gases.

Since natural graphite is not a pure chemical substance and is derived from different sources, its density is variable, but probably lies between the extreme values 2.015 and 2.583. Rammelsberg gave the value 2.17 to 2.32; Moissan found the value for artificial graphite to lie between 2.0 and 2.25; Dewar found that the density of an artificial graphite which was 2.099 at 17° C. became 2.1302 at the temperature of liquid air. According to Streintz the density of strongly compressed graphite may rise as high as 3.0. Arsem would define graphite by its density: 2.25 to 2.26.

The coefficient of linear expansion of graphite between -190° C. and +17° C. is 0.0000244. Fizeau, however, found for natural graphite the values 0.00000786 at 40° C. and 0.00000796 at 50° C. The compressibility of graphite between 100 and 500 atmospheres is 3×10^-6 vol./atm.

The specific heat of graphite, like that of other non-metals, increases with temperature, but becomes approximately constant in the neighbourhood of 1000° C., and then gives a value for the atomic heat = 5.6.

The following results were obtained by H. F. Weber:

Temp.° C.	Specific Heat	Atomic Heat.	Temp.° C.	Specific Heat	Atomic Heat.
-50°	0.114	1.37	202°	0.297	3.56
-10.7°	0.144	1.73	249°	0.325	3.90
+10.8°	0.160	1.93	642°	0.445	5.33
61.3°	0.199	2.39	822°	0.454	5.45
138°	0.252	3.03	977°	0.467	5.60

Behn has extended the values to lower temperatures, and gives the following interpolation formula:

C = 0.161 + 0.000628t – 0.000000112t².

Values calculated from this formula agree with Weber's results; but Dewar has obtained the following smaller values at low temperatures:

Temperature interval	Specific Heat.
18° to -78° C	0.1341
18° to -188° C	0.0948
-78° to -188° C	0.0599
-188° to -252.5 ° C	0.0133

In contradistinction to diamond graphite is a good conductor of heat and electricity. Specimens from different localities vary, however, in their conductivity. The following formula expresses the thermal conductivity of graphite:

K = 0.0384 - 9×10^-5t + 9.3×10^-7t².